The project involves two different platelet enzymes: 1) Platelet myosin light chain kinase: Phosphorylation of this enzyme by cAMP-dependent protein kinase decreases the activity of this enzyme. This might explain one way in which a rise in cAMP would act to inhibit platelet release: 2) Platelet protease: A Ca 2 ion-dependent protease which degrades human platelet myosin and smooth muscle myosin kinase has been purified. This protease might play an important role in the turnover of platelet contractile proteins.